Among wiring boards used in electronic equipment, a flexible printed-circuit board has excellent features such as flexibility, thinness, and lightness of weight. Among the flexible printed-circuit boards, those having a two-layered structure in which polyimide used for forming a base film and copper used for forming a conductive layer are bonded to each other without use of an adhesive has excellently heat-resistance, and therefore is demanded increasingly.
Methods for manufacturing substrates of the above-mentioned two-layered-structure flexible printed-circuit boards include plating, sputtering, casting, and laminating. Among these methods, the plating and the sputtering exhibit greater freedom in selecting a polyimide and easier manufacture.
The plating is a method including a step of forming a conductive thin seed layer on a polyimide film by electroless plating, and a subsequent step of forming a conductive layer of copper. The sputtering is a method including a step of forming a conductive thin seed layer on a polyimide film by sputtering and a subsequent step of forming a conductive layer of copper by electroplating.
For the foregoing plating methods, the semiadditive process is proposed. In the semiadditive process, to obtain intimate adhesion of the electroless copper plated layer with an interlayer insulator, conventionally recesses and projections are formed on a surface of the insulator by applying a chemical agent containing an alkaline permanganic acid solution and the like, and a plated layer is formed on the surface with the recesses and projections.
On the other hand, design pitches have been narrowed with the recent trend of size reduction and improved performance of electronic equipment, and accordingly, higher signal frequencies are used.
In the case where an electroless copper plated layer is formed on a surface with recesses and projections as described above, copper etching residues tend to remain after pattern formation by etching, and particularly in the case where the pattern pitch is narrow, there arises a problem that ion migration and other troubles tend to be induced by etching residues.
Further, when a clock frequency of current running through an electric conductor increases, current runs only in the vicinity of a surface of the conductor due to the skin effect, and here, in the case where recesses and projections are present at an interface between the plated copper layer and the insulator as described above, there arises a problem that signal transfer is retarded in the vicinity of the interface with the recesses and projections.
As an insulator used in the flexible board, a polyimide-based resin is used, and the polyimide substrate has to be brought into close contact with a resin of a bonding sheet, a photosolder resist and the like. However, the contact thereof with the resin greatly varies with the kind of the resin of the counterpart due to the properties of polyimide, and adhesion cannot be achieved at all when the resin is of one of certain specific kinds. Therefore, there has been a problem that the material for the use is limited.
As a method for processing a copper surface to improve the adhesion thereof with a resin, a blackening process (formation of copper oxide using strong-alkali aqueous solution) is performed in some cases. In this method, however, recesses and projections occur on an interface, which is expected to result in problems like those described above.
Further, since polyimide generally provides poor adhesion to a metal, a substrate manufactured by the aforementioned plating or sputtering method has a problem that a polyimide film provides poor adhesion to a seed layer and a copper layer. To cope with this, the following patent publication 1 has proposed that a surface of polyimide is subjected to plasma treatment so that the adhesion thereof to the seed layer and the copper layer is improved.